The invention relates to a receiver which is used in a mobile communications apparatus such as a personal handy phone, a portable phone, and a pager, to which the direct conversion system is applied.
Recently, radio communication apparatuses such as a personal handy phone, a portable phone, a pager, and a cordless phone have become widespread. As the modulation system for such radio communications apparatuses, usually, the frequency modulation, the phase modulation, the frequency shift keying, or the like is used. One of known receivers to which such a modulation system is applied is a direct conversion receiver having a configuration which can be easily integrated and which is suitable for miniaturization and lightening.
Such a direct conversion receiver of the prior art is configured as disclosed in, for example, Japanese patent publication (Kokai) 4-137927. An example of such a receiving circuit of the prior art is shown in FIG. 12. FIG. 12 is a block diagram showing the main receiving circuit portion of a direct conversion receiver of the prior art.
Hereinafter, the configuration of a receiving circuit of a direct conversion receiver of the prior art to which the frequency shift keying (abbreviated as "FSK") is applied will be described with reference to FIG. 12. In FIG. 12, reference numeral 1 designates a receiving antenna which receives a radio signal; 2, a received signal amplifier which amplifies the received signal from the receiving antenna 1 and outputs a modulated signal 3; 4, a first local oscillator which generates a first local signal 5 of a frequency that is approximately equal to the carrier frequency of the received transmission signal; 6, a 90-degree phase shifter which rotates the phase of the first local signal 5 by 90 degrees; 7, a first signal mixer which mixes the modulated signal 3 with the first local signal 5; and 8, a second signal mixer which mixes the modulated signal 3 with the signal that is obtained by phase-shifting the first local signal 5 by 90 degrees.
Furthermore, reference numeral 32 designates an I-low-pass filter which allows only an I baseband signal (in this example, low-frequency components) that is included in the output signal of the first signal mixer 7 and in phase with the received signal, to pass therethrough, thereby outputting an I-baseband signal 17, and in which the cut-off frequency is not varied. The reference numeral 33 designates a Q-low-pass filter which allows only a Q baseband signal (in this example, low-frequency components) that is included in the output signal of the second signal mixer 8 and phase-shifted by 90 degrees, to pass therethrough, thereby outputting a Q-baseband signal 18 that is perpendicular to the I-baseband signal 17, and in which the cut-off frequency is not varied. The reference numeral 11 designates detecting means for adding the received I-baseband signal 17 with Q-baseband signal 18, and conducting detection to output a demodulated signal 19, and 12 designates received data detecting means for judging the demodulated signal 19 to output the received data.
The operation of the receiving circuit of the direct conversion receiver of the prior art will be described with reference to the same figure or FIG. 12. A radio signal which is received by the receiving antenna 1 is amplified by the received signal amplifier 2, and then output as the modulated signal 3 to the first and second signal mixers 7 and 8. On the other hand, the first local signal 5 generated by the first local oscillator 4 is mixed in the first signal mixer 7 with the modulated signal 3, and then output to the I-low-pass filter 32. At the same time, the first local signal is rotated in phase by 90 degrees by the 90-degree phase shifter 6, then mixed in the second signal mixer 8 with the modulated signal 3, and then output to the Q-low-pass filter 33.
The I-baseband signal 17 and Q-baseband signal 18 which have passed through the I- and Q-low-pass filters 32 and 33 are added with each other in the detecting means 11, and then the summed signal is waveform-shaped, detected, and demodulated to be output as the demodulated signal 19 to the received data detecting means 12. The received data detecting means 12 samples the demodulated signal 19, detects received data sent from a transmission station (not shown), and outputs the detected received data to other processing means such as a CPU.
With the recent rapid propagation of small and light mobile communications apparatuses such as a personal handy phone, a pager, a cordless phone, and a portable phone, the demand for a direct conversion receiver which is suitable for integration is growing. On the other hand, with the expansion of the number of users of mobile communications apparatuses, also the communications capacity of the mobile communication is rapidly increased. In order to cope with such increases of the demand for mobile communications apparatuses, it is urgently required to increase the transmission data rate (hereinafter, referred to as "transmission rate") and narrow the band of the communication frequency.
In order to satisfy the requirement, in a pager, for example, a system in which a plurality of transmission rates are variable (the transmission rate can be dynamically changed among plural defined ones) is specified in the advanced radio paging system standard (RCR STD-43).
In the case where a single and fixed transmission rate is used as in the prior art, there arises no problem even when the cut-off frequency of a baseband filter which is used in the direct conversion receiver of the prior art is fixed (hereinafter, th term "baseband filter" does not mean a filter of a particular kind or an I-, Q-, high, or low-pass filter, but is used as a general term for filters including those of such kinds). In other words, for a fixed transmission rate, it is possible to use an optimum baseband filter having a cut-off frequency fixed to a value at which adjacent channel interference is small in level and the reception sensitivity is optimum.
In the case where such a direct conversion receiver of the prior art is used in a system wherein a plurality of transmission rates are variable as specified in the advanced radio paging system standard, however, there arises the following problem. When the transmission rate is changed, such a baseband filter used in the prior art direct conversion receiver is deviated from the optimum state, so that adjacent channel interference occurs and the reception sensitivity is lowered.